The long-term goal of this research is to describe the auditory processes that allow us to hear in background noise. Each sensory system shows a hierarchical organization with functional specializations for separating biological signals from less important backgrounds. Our studies have characterized neural response types within the central auditory system that can be interpreted as mechanisms for extracting auditory signals from background noise. These neurons appear to connect in series across the major auditory nuclei to create parallel pathways with exceptional noise-cancellation capabilities. Our proposed studies will address the following questions: What are the special physiological properties of central auditory neurons that improve the encoding of signals in noise (local processing)? Does the improved representation remain functionally segregated as it ascends the auditory system (parallel processing)? Is the neural representation further enhanced when it reaches higher structures (series processing)? How do descending pathways influence the quality of the representation (efferent processing)? Answers to these questions will be pursued with a combination of single-unit electrophysiological techniques and animal psychophysical paradigms. Ties between physiological and behavioral results will be strengthened by performing single-unit studies in awake preparations and using psychophysical procedures that incorporate the essential stimulus conditions of physiological experiments. In addition to experiments in cats that build directly upon our previous studies, the mouse will be introduced as a model for auditory signal processing. Cross-species comparisons of our physiological and behavioral results will distinguish general auditory processes from species-specific specializations. A better understanding of the brain's signal processing solutions will lead to improved designs for assistive listening devices that now show poor performance in noisy environments.